1. Field of the Invention
The present invention relates to a light-emitting element as typified by an organic EL element and a display device having the light-emitting element.
2. Related Art
In recent years, a display device having a light-emitting element as typified by an organic EL (Electro Luminescence) element has been developed and expected to be widely used with taking advantages of a high quality image, a wide viewing angle, and a slim and lightweight shape due to that the display device is a self-luminous type. The “EL element” refers to a light-emitting element that uses a principle that an electroluminescent layer emits light that is interposed between a pair of electrodes by applying current to an anode and a cathode. As the light-emitting element, for example, a so-called top emission light-emitting element having a transparent electrode at a side of an opposite substrate to emit light to the side of the opposite substrate is known. FIG. 17 illustrates a cross-sectional view of the top emission light-emitting element. In FIG. 17, reference numeral 1 denotes a substrate; 2, an electrode; 3, a hole transporting layer; 4, a light-emitting layer; 5, an electron injecting layer; 6, a transparent electrode; 7, a moisture barrier layer; and 8, an antireflection layer. The light-emitting element utilizes light that is radiated when an exciton that is generated by the recombination of electrons injected to the light-emitting element 4 from the transparent electrode 6 via the electron injecting layer 5 and holes injected to the light-emitting layer 4 from the electrode 2 via the hole transporting layer 3 returns to the ground state.
The light-emitting element that extracts light from a top side (opposite substrate side) such as a top emission light-emitting element is required to use the transparent electrode 6 as an opposite electrode. For example, an indium tin oxide (ITO) or the like is used, in which case there is a problem that light extraction efficiency is deteriorated since there is large difference in refractive indexes of the transparent electrode 6 and air around the transparent electrode 6 (patent documents 1 and 2).
Further, a light-emitting element including an organic compound as its main constituent is subject to be deteriorated mainly due to moisture or oxygen. As deterioration due to the moisture or oxygen, luminance is partly lowered or a non emission region is produced. In order to prevent the deterioration, technique of forming a passivation film (moisture barrier layer 7) such as a SiN film for moisture barrier over the transparent electrode 6 is known (a patent document 2). However, there is a problem that light-extraction efficiency is deteriorated since there is large difference in refractive indexes between the SiN film and air even in the case that the passivation film (moisture barrier layer 7) such as a SiN film for moisture barrier is formed over the transparent electrode 6 (a patent document 2).
Technique of forming a film made from a material having a lower refractive index than that of the transparent electrode 6 as an antireflection film 8 in a signal layer or a multi-layer over the transparent electrode 6 or the moisture barrier layer 7 is known toward the foregoing problems (patent documents 1 and 2).
Patent document 1: Unexamined Patent Publication No. 2003-303679
Patent document 2: Unexamined Patent Publication No. 2003-303685
The refractive index of the transparent electrode 6, for example, ITO, is approximately 1.9 to 2.0, whereas the refractive index of the moisture barrier layer 7, for example, a SiN film, formed over the transparent electrode 6 is approximately 2.1 to 2.3, which is higher than that of the transparent electrode 6. In the case that the moisture barrier layer 7 such as a SiN film is formed over the transparent electrode 6, there is still large difference in refractive indexes between the moisture barrier layer 7 and the antireflection layer 8 even when the antireflection layer 8 having a lower refractive index than that of the transparent electrode 6 is formed over the moisture barrier layer 7. Therefore, the reflectivity of an interface between the moisture barrier layer 7 and the antireflection layer 8 is further increased compared to the case that the moisture barrier layer 7 is not provided. Therefore, there is a problem that light extraction efficiency from the light-emitting layer is deteriorated with the increase of reflection loss of light at the interface.
In the case of forming the moisture barrier layer 7, there is a problem that peeling, cracking, and the like due to stress are often occurred, which leads to deterioration of manufacturing yields, lower reliability, and reduction of lifetime of a light-emitting element.